1. Field of the Invention
The present invention relates to a process for preparing a chlorinated toluene derivative which is chlorinated at its nuclear position or positions, and particularly relates to a novel co-catalyst which is used in the process for the preparation of a chlorinated toluene to form a product selectively chlorinated at para-position.
2. Brief Description of the Prior Art
Processes for the preparation of nuclearly chlorinated toluenes and the products prepared thereby have been recognized in the art as being extremely important industrial techniques and products. The chlorination reaction has been carried out, most commonly, in the presence of a Lewis acid catalyst such as antimony chloride, ferric chloride or aluminum chloride, to form monochloride and polychlorides and position isomers thereof together with additional formation of benzyl chloride under certain reaction conditions. The main products formed under conditions for the preparation of monochlorotoluene are o-chlorotoluene (hereinafter referred to as OCT) and p-chlorotoluene (hereinafter referred to as PCT), and the by-products include small amounts of m-chlorotoluene (hereinafter referred to as MCT), dichlorotoluene and benzyl chloride, the formation and quantity of the aforementioned by-products varying depending on the reaction conditions employed.
Among the main products, i.e. OCT and PCT, OCT is relatively useless but PCT is a very important industrial product and of high value in a wide range of fields as a material for various industrial products, agricultural chemicals and medical products. For this reason, it is important to increase the yield ratio of PCT to OCT in the preparation of monochlorotoluene from the economical standpoint of view. Any increase in the yield ratio of PCT relative to the total quantity of monochlorotoluene, even a mere 0.5% increase, is of great economic merit. Many efforts have hitherto been made to increase the yield ratio of PCT by selecting suitable catalyst and conditions for chlorination.
For example, U.S. Pat. No. 3,000,975 discloses a process wherein a chloride of titanium, tin or zirconium is used as the catalyst to obtain the result of PCT/OCT=23/75, and U.S. Pat. No. 3,226,447 discloses a process wherein a combined catalyst comprising a halide of iron, aluminum or antimony and an organic sulfur compound, such as mercaptoacetic acid, is used to obtain the result of PCT/OCT=31.5/38.0. Further improved results were obtained by using the catalysts disclosed in the following earlier Patent specifications, these known catalysts including a catalyst system comprising FeCl.sub.3 and S.sub.2 Cl.sub.2 as disclosed in Nederland Patent Application No. 6,511,488, a catalyst comprising PtO.sub.2 as disclosed in U.S. Pat. No. 3,317,617, a catalyst disclosed in French Pat. No. 1,491,143 which comprises an iron base catalyst combined with a co-catalyst made of an inorganic sulfur compound, such as SOCl.sub.2, CS.sub.2 or ZnS, and a catalyst disclosed in French Pat. No. 1,491,144 which comprises a Lewis acid catalyst combined with selenium or an organic sulfur compound, such as thiophene or dimethylthiophene.
Referring further to the prior publications, Japanese Patent Publication No. 34009/1975 discloses a catalyst system comprising iron combined with selenium, selenium oxide or selenium halide to obtain the result of PCT/monochlorotoluene=52.1 to 52.6%, Japanese Patent Laid-Open Publication No. 19631/1977 discloses a catalyst comprising a Lewis acid combined with a thianthrene co-catalyst to obtain the result of PCT/monochlorotoluene=50.5%, Japanese Patent Laid-Open Publication No. 19630/1977 discloses a similar catalyst containing polychlorothianthrene to obtain the result of PCT/monochlorotoluene=55.9%, and Japanese Patent Laid-Open Publications No. 44529/1978 and No. 87323/1978 discloses a similar catalyst containing a co-catalyst made of thianthrene compound having an electron attractive substituent and an electron donative substituent to obtain the result of PCT/monochlorotoluene=56 to 59%.
Furthermore, Japanese Patent Laid-Open Publication No. 5139/1981 discloses a phenoxthine compound having a hydrogen atom, an electron attractive group or an electron donor group at the position 2-, 3-, 7-, or 8-.
The yield ratio of PCT to OCT can, in fact, be improved according to the processes disclosed in the prior publications referred to above. However, these known catalyst include those which do not act to increase the yield ratio to the expected extent, which are too expensive to be used in the industrial scale, those containing a co-catalyst which cannot be easily synthesized, and those which can not increase the yield so substantially or which can not be readily available. For such reasons, difficulties are encountered in using them on an industrial scale, even though the selectivity to PCT may be improved satisfactorily by their use.